A brushless motor typically includes a permanent magnet rotor and a stator having three, normally delta connected windings. Motors of this type are usually driven from integrated circuits whose output stage, supplying the winding phases, has a full-wave three-phase bridge circuit formed from six power transistors of the bipolar or the MOS type. FIG. 1 shows an example of such a driver stage and a circuit diagram of a DC brushless motor connected thereto.
The most typical form of driver in use with motors of this type is the bipolar driver, whereby two phases are under power while the third is floating (having a bridge output in a high-impedance state: Hi Z) at any one time.
The powered phases are switched in a cyclic sequence which must be synchronized with the instantaneous position of the rotor. This position is detected by analysis of the back electromotive force (BEMF) of the floating phase or detected by sensors. The detection is usually made with bipolar type drivers.
For improved performance of the system, the phase supply should be optimized to operate the motor at the peak of its efficiency, which can be obtained by driving the phases while retaining a precise phase relationship between the current and BEMF of each phase (with the optional use of position sensors). In synchronous permanent magnet motors, like the DC brushless motor, torque is provided by the component of the stator current which is generating a magnetic field in quadrature with that generated by the rotor.
If i.sub.d and i.sub.q and the components of the stator current that generate the magnetic fields in a straight axis and in quadrature, respectively, with that from the rotor: peak efficiency is achieved when i.sub.d =0. In order to have the whole of the stator current generate the fields in quadrature, it is mandatory that the current of each coil be driven in phase with its BEMF.
Heretofore, the control of motor position, and hence the accuracy of the motor operation, has been limited to 60 electrical degrees. In general, one signal occurs every 60 degrees which is used as an indicator of the instantaneous position of the motor (as by sensing the BEMF of the coils or using purposely designed position sensors), and this information is utilized to adjust the drive voltages or currents for the new position. This results in a clipped drive waveform (FIG. 2) which is apt to originate acoustic and electromagnetic noise in the motor. Such noise is problematic in disk drive motors, CD motors etc., because this generated noise can interfere with the reading or writing signals of the device, causing incorrect results and reduced performance of the device.